Landlords need to be aware both of specific legislation and of general responsibilities.
While there is specific legislation requiring, for example, annual gas safety checks on rented properties there is no similar legislation covering the circumstances you refer to.
However you still have a general responsibility to ensure that the property you rent out meets (what a reasonable person would deem to be) appropriate standards of safety. So if your property has a garden pond, and you rent it out to a family with young children, it would be appropriate to ensure that a fence was placed around the pond. (If you didn't, and a tragedy occurred, a court might hold that you were at least partly to blame for it).
I very much doubt that there are many circumstances where you could be reasonably expected to fit safety glass to a shed or greenhouse but if there was the possibility of an excited child running headlong through a greenhouse window, it might be wisest to place some sort of barrier to prevent such a thing happening. (The problem of advising on a situation where there is no specific legislation in place is that courts only decide whether a landlord has met his/her responsibilities if a case goes to court after an accident has already happened, so it's necessary to guess as to the decision a court might come to).